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OS5B-4:FLUID FLOW CONTROLLED GAS HYDRATE AND TEMPERATURE DISTRIBUTION IN THE POCKMARK: A CASE STUDY ON THE CONTINENTAL MARGIN OF NIGERIA
发布时间:2014-07-28
Jiangong WEI1, Gerhard BOHRMANN1, Livio RUFFINE2, Thomas PAPE1, Vincent RIBOULOT2, Jean-Louis COLLIAT3, Alexis DE PRUNELE2, Bernard DENNIELOU2, Sebastien GARZIGLIA2, Tobias HIMMLER2, Tania MARSSET2, Carl PETERS1, Abdulkarim RABIU4, Nabil SULTAN2
1. MARUM, University of Bremen, Germany;2. IFREMER, Département des Géosciences Marines, France; 3. TOTAL, France ; 4. NIOMR, Nigeria Institute for Oceanography and Marine Research, Nigeria
Pockmarks are circular or elongated seafloor depressions which are believed to be the expressions of the fluid flow from the subsurface. In deep-sea areas, gas hydrates can form in the sediment from light hydrocarbons brought along with the fluids and may significantly influence the pockmark evolution. A prerequisite to study interactions of gas hydrates with pockmark formations is to precisely determine gas hydrate distributions in the pockmark deposits. In this paper, we document gas hydrate and temperature distributions in a pockmark at the continental margin of Nigeria and discuss its control factors. A comprehensive investigation of the so-called pockmark A was conducted during the ‘Guineco-MeBo’ cruise in 2011. This pockmark is situated at water depths of approx. 1140m and is about 550m in seafloor diameter. Seven sediment cores with the total length of 180m were drilled in this pockmark using the mobile sea floor drill rig MeBo of the MARUM. We performed infrared (IR) thermal scanning of the cores immediately after recovery and chloride concentration measurements of pore waters in order to map gas hydrate distributions in the sediments. The estimated gas hydrate-bearing sediment intervals as revealed either by low temperature (IR) or, both positive and negative chloride anomalies caused by hydrate formation/dissociation are generally consistent with each other. Comparison of the results from MeBo coring with those from seismic surveys in the frame of the ERIG- 3D cruise in 2008 showed that gas hydrate-bearing sediments were represented by high amplitude reflectors in the seismic profiles. In general, gas hydrate distributions exhibited two major features: 1) presence of shallow hydrates in the pockmark center and deeper hydrates in its periphery; 2) two distinct hydrate-occupied fractures in the subsurface of the center. In situ sediment temperature measurements at ten selected sites were also conducted to study the geothermal structure of pockmark A. While the background geothermal gradient was about 72°C/km off the pockmark, steep gradients (198-330°C/km) were measured in a restricted area in the pockmark center. This location corresponds with the seafloor origin of gas flares in the water column as observed during hydro-acoustic surveys. We conclude that the gas hydrate and temperature distribution in pockmark A were controlled by fluid flow, especially gas flow composed of methane migrates from the deep to the shallow sediments. In the center of pockmark A, fractures are generated when the excess pore water pressure exceeds the hydrostatic effective stress. Rapid fluid transport through the highly permeable fractures to the water column leads to shallow gas hydrate formation and an increase in sediment temperature by heat convection. At the periphery of the pockmark, less intensive fluid flow dominated by diffusion only allows for deeper hydrate accumulations and lower geothermal gradients.
1. MARUM, University of Bremen, Germany;2. IFREMER, Département des Géosciences Marines, France; 3. TOTAL, France ; 4. NIOMR, Nigeria Institute for Oceanography and Marine Research, Nigeria
Pockmarks are circular or elongated seafloor depressions which are believed to be the expressions of the fluid flow from the subsurface. In deep-sea areas, gas hydrates can form in the sediment from light hydrocarbons brought along with the fluids and may significantly influence the pockmark evolution. A prerequisite to study interactions of gas hydrates with pockmark formations is to precisely determine gas hydrate distributions in the pockmark deposits. In this paper, we document gas hydrate and temperature distributions in a pockmark at the continental margin of Nigeria and discuss its control factors. A comprehensive investigation of the so-called pockmark A was conducted during the ‘Guineco-MeBo’ cruise in 2011. This pockmark is situated at water depths of approx. 1140m and is about 550m in seafloor diameter. Seven sediment cores with the total length of 180m were drilled in this pockmark using the mobile sea floor drill rig MeBo of the MARUM. We performed infrared (IR) thermal scanning of the cores immediately after recovery and chloride concentration measurements of pore waters in order to map gas hydrate distributions in the sediments. The estimated gas hydrate-bearing sediment intervals as revealed either by low temperature (IR) or, both positive and negative chloride anomalies caused by hydrate formation/dissociation are generally consistent with each other. Comparison of the results from MeBo coring with those from seismic surveys in the frame of the ERIG- 3D cruise in 2008 showed that gas hydrate-bearing sediments were represented by high amplitude reflectors in the seismic profiles. In general, gas hydrate distributions exhibited two major features: 1) presence of shallow hydrates in the pockmark center and deeper hydrates in its periphery; 2) two distinct hydrate-occupied fractures in the subsurface of the center. In situ sediment temperature measurements at ten selected sites were also conducted to study the geothermal structure of pockmark A. While the background geothermal gradient was about 72°C/km off the pockmark, steep gradients (198-330°C/km) were measured in a restricted area in the pockmark center. This location corresponds with the seafloor origin of gas flares in the water column as observed during hydro-acoustic surveys. We conclude that the gas hydrate and temperature distribution in pockmark A were controlled by fluid flow, especially gas flow composed of methane migrates from the deep to the shallow sediments. In the center of pockmark A, fractures are generated when the excess pore water pressure exceeds the hydrostatic effective stress. Rapid fluid transport through the highly permeable fractures to the water column leads to shallow gas hydrate formation and an increase in sediment temperature by heat convection. At the periphery of the pockmark, less intensive fluid flow dominated by diffusion only allows for deeper hydrate accumulations and lower geothermal gradients.
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